In the continuous treatment of many materials, there are temperature and reactant chemical gradients that occur such that uniform treatment of the material across the cross-section of the treatment vessel is not obtained. Typically, spent liquid is withdrawn through an annular screen at the interior of the vessel, and treatment liquid is introduced into a central pipe within the vessel. When hot treatment liquid is utilized, the temperature near the wall of the vessel will be much less than in the middle, and also the chemical concentration of the liquid near the wall will be much less than in the center.
Non-uniformity in the continuous treatment of material takes place in a wide variety of continuous processes, but is especially prevalent in the cooking of comminuted cellulosic fibrous material (e.g. wood chips), as in the kraft cooking, sulfite cooking, or solvent pulping processes. Such non-uniformity can result in a non-uniform distributing of residual lignin content (e.g. measured by Kappa No.), a lower average viscosity, and lower physical strength characteristics (e.g. tensile, burst, tear, etc.) of the final product (e.g. paper or paper board) produced.
A number of techniques have been utilized commercially in order to minimize such non-uniformity of treatment in the digesting of wood chips, particularly in the kraft pulping industry. One such technique is the operation of a latter portion of the reaction zone in a counter-current manner. However this does not eliminate the non-uniformity in chip and liquor distribution.
A number of other techniques have also been employed to enhance uniformity in continuous digesters. Typically, one or more annular screens are installed in a continuous digester for the purpose of extracting circulating liquids. Various arcuate segments of the screens are operated by others are inactive in order to withdraw liquid in a uniform manner. During normal operation it is not uncommon for screen segments to become partially plugged with either fines or over reacted particles. In order to overcome this plugging, in addition to alternate switching off or resting of one or more screen segments, sometimes backflushing of withdrawn liquid is practiced, to make a positive discharge of the fines or soft particles from the screen segments. The withdrawn liquid is typically used in backflushing although it is possible to use pressurized filtrates from other parts of the pulping process.
Typical of prior art constructions are German patent 893,441, and U.S. Pat. Nos. 2,414,062, 2,695,232, and 4,547,264. The German patent describes milled extraction screen plates which are commonly used in continuous digesters. The plates consist of multiple horizontal rows of vertically milled slots relieving toward the back. U.S. Pat. No. 2,414,062 covers a teohnique for fully heating the chip column via flow from an inlet which extends around some fraction of the circumference, through the downflowing column of chips, to an extraction sieve located opposite the inlet or inlets. Temperature distribution problems arise from utilizing this technique to perform the function of heating the chips to full reaction temperature, and thus the disclosure of U.S. Pat. No. 2,695,232 was developed. This disclosure shows the utilization of a central pipe for introducing treatment liquor into the column, and what is now considered to be a standard method of heating. This patent specifically distinguishes itself from the techniques disclosed in U.S. Pat. No. 2,414,062, which was not able to always successfully perform the function of heating the chips to full reaction temperature. The system in U.S. Pat. No. 4,547,264 was developed to reduce the temperature distribution problems existing at that time, however it results in a radial temperature distribution problem.
The invention relates to a combination of techniques wherein a moving multi-point injection of heated and fortified liquor is utilized, using limited flows at the periphery of the vessel, to significantly reduce radial temperature distributions which exist in today's digesters.
According to the present invention, the uniformity of chips and the continuous digesting thereof is enhanced by introduction of treatment liquid opposite withdrawal segments of screens in a continuous digester. While the treatment liquid can be introduced through a screen opposite one or more screens through which withdrawal is taking place, it is preferred that the treatment liquid be introduced through slots or nozzles disposed in land areas adjacent the screens.
According to one aspect of the present invention, a method of continuously uniformly treating material in a vessel is provided. The vessel has at least one annular screen, divided into at least four arcuate screen segments, disposed around the internal periphery of the vessel, and the method utilizes a first liquid having treatment properties, the first liquid ultimately becoming spent after remaining in contact with the material to produce a second, spent, liquid. The method comprises the steps of: (a) Withdrawing the second liquid through at least a first of the arcuate screen segments. (b) While practicing step (a), introducing first liquid through or adjacent at least a second of the arcuate screen segments, opposite the first screen segment. (c) Terminating steps (a) and (b). (d) Withdrawing the second liquid through at least a third of the arcuate screen segments. (e) While practicing step (d), introducing first liquid through or adjacent at least a fourth of the arcuate screen segments, opposite the third screen segment. (f) Terminating steps (d) and (e). (g) Continuously repeating steps (a)-(f) for all of the screen segments. And, (h) continuously introducing first liquid into the center of the vessel, adjacent the level of the annular screens in the vessel. Each arcuate screen segment is preferably separated from the others by a land, and each arcuate screen segment has a liquid introduction device adjacent to it, but no in liquid communication with it. Steps (b), (e) and (g) are then practiced by introducing the first liquid through the liquid introduction device adjacent the screen segment.
The method may employ for example, eight arcuate screen segments, and during the practice of steps (a), (d), and (g), second liquid is withdrawn through three adjacent screen segments at the same time (a middle and two side segments), while steps (b), (e), (g) and (h) are practiced by introducing first liquid only through the liquid introduction device associated with the screen segment opposite the middle of the withdrawing screen segments, and through a central pipe. The second liquid withdrawn during the practice of steps (a), (d), and (g) is preferably treated to increase at least one of the temperature and treatment chemical concentration thereof, to produce first liquid, and then re-introducing the first liquid in the practice of steps (b), (e), and (g). As earlier indicated, the material being treated is preferably--although not necessarily--comminuted cellulosic fibrous material, and the first liquid is the digesting liquid having a desired digesting temperature and chemical content and concentration.
According to another aspect of the present invention, apparatus is provided for treating material with a first, treatment liquid which becomes spent after prolonged contact with the material to produce second liquid. The apparatus comprises: A generally upright vessel. At least one annular screen extending around the interior periphery of said vessel at a predetermined height thereof. The screen having at least four distinct arcuate screen segments, the segments separated by lands, and each of substantially the same arcuate extent. Means for withdrawing the second liquid through at least a first of the arcuate screen segments. Means for introducing first liquid through or adjacent at least a second of the arcuate screen segments, opposite the first screen segment, simultaneously with the withdrawal of second liquid through the first screen segment. Control means for controlling the withdrawing and introducing means to periodically change the screens through which withdrawal takes place, and through which, or adjacent to to which, introduction of first liquid takes place. And, a central vertical pipe for introducing first liquid into the vessel adjacent the annular screen, and preferably two vertically spaced annular screens are provided. Means for introducing the first liquid preferably comprises a liquid introduction device adjacent each of the screens, the liquid introduction device comprising a horizontally elongated slot or a plurality of nozzles, which can be easily vertically spaced from the segment with which it is associated, or located in land area between the screen segment with which it is associated and adjacent screens segments.
It is the primary object of the present invention to provide a method and apparatus for simply yet effectively increasing the uniformity of treatment of the material with treatment liquid, particularly the digestion of wood chips or the like or produce paper pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.